In a wave gear device, an external-teeth area in a cup-shaped or silk-hat-shaped flexible externally toothed gear is flexed into a non-circular shape, e.g., an elliptical shape by a wave generator, whereby the external teeth of the flexible externally toothed gear are partially meshed with the internal teeth of an annular rigid internally toothed gear. When the wave generator is rotated by a motor, the meshing positions of the two gears move circumferentially, and relative rotation occurs between the two gears, the rotation corresponding to the difference in the number of teeth between the two gears. When the wave gear device is used as a reducer, commonly, the rigid internally toothed gear is fixed in place so as to not rotate, high-speed rotation inputted from the motor to the wave generator is greatly reduced, and the reduced rotation is outputted from the flexible externally toothed gear.
Patent Document 1 proposes a wave gear device comprising a cup-shaped flexible externally toothed gear. In the wave gear device disclosed in this document, the face width of the external teeth of the cup-shaped and silk-hat-shaped flexible externally toothed gear is reduced for the purpose of suppressing poor meshing of the flexible externally toothed gear with the rigid internally toothed gear, whereby a proper state of meshing can be maintained between the flexible externally toothed gear and the rigid internally toothed gear. To enable the external-teeth area to flex easily, a thin portion is formed in the portion of the flexible externally toothed gear that is adjacent to the external-teeth area in the cylindrical cup part, enabling the entire external-teeth area to flex appropriately and thereby improving the state of meshing between the two gears.